This invention is directed to a method for connecting leadless chip packages by employment of ultrasonic welding for attaching metallic conductors from pads on the leadless chip carrier to related pads on the associated printed wiring board.
A leadless chip carrier package is one which carries pads on the lower surface thereof. These pads are internally connected in the package to the integrated circuit chip housed and protected therein. Attachment is accomplished by placing this package onto a printed wiring board which has corresponding solder pads thereon. The leadless chip package is secured in place and electrically connected in one step by reflow soldering of the pads together by vapor-phase soldering. Such attachment is useful in many cases, but when the chip becomes electrically more complicated to result in a greater number of leads to result in a high density of pads around the lower periphery of the leadless chip package, attachment becomes more difficult. When the package is of higher density, that is with more connections than about 60 pads, the traces on the printed circuit board connecting the pads are close together so that solder bridging between traces and pads is more likely to occur because of the close spacing.
Making the leadless chip packages larger to provide more spacing between the pads accentuates other problems. In complicated electronics, the space on a printed wiring board is at a premium so that use of larger than necessary packages is wasteful of needed board space. Larger packages, whether created by more complicated electronics and greater number of required circuit connections or created by greater spacing between the pads to reduce risk of solder bridging, accentuates the problem of thermal expansion. The leadless chip packages are of a different material than the printed wiring board so that, upon temperature change, the dimensional differences produce strains in the leadless package solder pads. When these strains become excessive, cracked solder pads result. This leads to the step of building the solder pads to a higher level so that strains caused by temperature change are absorbed in the pad by resilient or ductible deflection.
It is thus seen that the conventional solder reflow attachment of leadless chip packages experiences problems when the number of pads to be connected increases so that the pads and circuit traces on the board are in a high density arrangement. Thus, there is need for a method for connecting high density packages, particularly high density leadless chip carrier packages to a corresponding printed wiring board.